In the movie "Interstellar," a landing craft explores
a planet near a black hole that has a mass 100 million
times that of the Sun. The crew spend just a few
minutes on the planet, but when they return to the main ship, 23 years have elapsed. I've waited years. It's 23 years, four months, eight days. The idea of time dilation by going into deeper gravitational fields is actually pretty accurate. First theorized by Einstein in his theory of relativity,
it's known as time dilation. The very bizarre thing is that when you're in a place
where gravity is stronger, time moves more slowly. But that's not all. Another way time dilation occurs is through your relative
movement through space. If you have two observers that have clocks that tick at exactly the same rate, and then one of those observers moves away at a very high speed, the moving observer's clock
will tick more slowly. Their time will literally pass slower. And a lot of people are confused by this 'cause they think it might
be a mechanical thing. It's the clock. But no, it
is in fact, literally time. It doesn't matter what the clock is. It can be a clock. It
could be your heartbeat. It's time. This is even true for
someone who is walking compared with someone sitting at a table. Time moves slower for the
person who is walking. Similarly, for
gravitational time dilation, someone who is at sea level compared with someone on a mountain will also experience
time at a slower rate. That's because gravity is stronger when you're closer to the
Earth's center of mass. But in these examples, the amount of time
dilation is so minuscule that it's totally unnoticeable. Apart from a few examples, humans don't deal with fast
enough speeds or enough gravity to experience a significant
amount of time dilation. But that may not matter. Because atomic clocks are
reaching a sensitivity that can measure even the
tiniest amount of time dilation. I think that's the place where relativity will start to become far more important. It's not because we can go so fast. I mean, I wish we could go fast, but I think the real place will be because we can measure things with an incredible precision. That's amazing. That level of precision will enable us to use time in ways we never thought possible. Like monitoring tectonic plates, large movements deep
below Earth's surface, and climate change. Atomic clocks could even
lead to new discoveries regarding the nature of time itself. I think that's one of the
really exciting things about this, is that we're
really exploring the unknown or able to sense effects that no one has been able to see before using the precision of the clocks. In 2010, two ultra-precise, synchronized atomic clocks
were put next to each other. These clocks were so
precise that they would lose about one second over the
history of the entire Earth. Then one of them was raised by just 33 centimeters. The result? The clock that was higher
up began to tick faster. But nothing was wrong with the clocks. It was actually time that
sped up for the higher clock, relative to the lower clock due to the tiniest decrease in gravity as the clock moved higher up. Ultra-precise clocks measured
the effects of relativity side by side, being changed by that much. Absolutely amazing, fantastic experiment. Atomic clocks were used in the '70s for that very purpose, testing relativity and other
scientific experiments. They flew some atomic clocks on airplanes around the world, compared the time before
and after doing that. What they found confirmed
their predictions: Time slowed down for the moving clocks. The technology ended up
being integral to devices that we depend on every day, GPS. Global navigation satellite systems depend on atomic clocks to
help determine where you are. The time it takes for a signal traveling at the speed of
light to reach your phone will give you the distance. Using four or more of them, you can triangulate your position. I really think of the atomic clocks as constantly working in the
background, you don't see it, but atomic clocks are really important for having the whole thing run. The invention of GPS
meant that time dilation suddenly went from being a cool theory to something that needed to be
dealt with on a daily basis. Due to the speed at which the satellites are orbiting the Earth,
14,000 kilometers an hour, and the lower gravity in space, they are actually
experiencing time differently relative to us on Earth. If you add everything up, the clocks change by
about 40 microseconds, 40 millionths of a second of a day, which sounds like nothing. But if you did not take that
relativity into account, every day, your position on your receiver would say that you were 6 miles off from where you actually are. So it would very quickly
be completely useless. These dramatic effects of relativity are central to the work done here at the National Institute
of Standards and Technology or NIST in Boulder, Colorado. NIST is a leader in the development of optical atomic clocks. These clocks use lasers to control atoms and measure their oscillations. Atomic clocks are currently the most stable and accurate way of measuring time intervals. And like any other clock, at
the heart of an atomic clock is a very stable oscillator. So you think about a grandfather clock. It has a pendulum that serves
as the reference oscillator. It oscillates at one cycle per second. An atomic clock is very similar. So at the heart of an atomic
clock is an oscillator, which is the atom itself. Instead of having a
bunch of gears and levers and springs and things,
it's a series of lasers. Atoms trapped in an electric field receive a series of pulses, which aid in controlling and
measuring their oscillations. Instead of one tick per
second, these clocks oscillate or tick a million
billion times per second, allowing for much more precision. So it's a really small inaccuracy. It's hard for anyone to conceptualize that level of performance. These clocks are so reliable that it would take more than
double the age of the universe, so around 30 billion years,
to drift by even a second. That precision means relativity can also be measured
with incredible accuracy. So if you move a clock up
by 1 centimeter of height, it will tick faster by this
one part in 10 to the 18. And now we can detect that kind of change. Although this kind of precision could be problematic since atomic clocks keep the official time that synchronize devices
all over the globe. That seems like a huge problem, I mean how do you decide
where to place the clocks if the tiniest change in gravity causes time to slow down or speed up? That is a good point and it's a problem that
we're sort of anticipating. Now the clocks are getting so good that we don't know their
gravitational potential well enough to define time globally
at that same level. It does seem like a simpler environment somewhere out in space, farther away from these local
gravitational variations. The problem of determining
standard time aside, this 1-centimeter level precision has the potential to revolutionize the way things are measured, using clocks to calculate
height and gravity based on the amount of time dilation. This is particularly useful in geodesy, the study of the Earth's
shape, orientation in space and its gravity field. We're studying the motion
of the crust of the Earth, the plate tectonics, the tides,
the polar motion of Earth. There are many, many effects that observe and that can be explained. For instance, in Sweden and Canada, it's rising on the order of
1 to 2 centimeters per year. That's quite a lot. And on another part of our planet, there's actually a downward land movement. So somewhere it's going up
and somewhere it's going down. And such effects, of
course, change everything like the global sea level, earthquakes. They changed even the rotation
of the Earth and much more. Understanding these movements can help aid in
understanding climate change, natural hazards and
construction of major structures like bridges and roads. If atomic clocks were used in this field, they could in theory give
you height measurements that are sensitive to just 1 centimeter. A level of accuracy GPS is not capable of. And because current methods
utilize GPS and a gravimeter for determining gravity at a single point, it doesn't always give you a clear picture of what's going on. Well, I'm from Germany, so
let's give you an example. In Germany, we're proud to
have our highest mountain. It's called Zugspitze. It's
almost 3,000 meters high. But also this mountain
has permafrost underneath, which is now melting. And that actually gives rise
to a shrinking of the mountain. So the top of the mountain
is probably decreasing by also 1 or 2 centimeters per year. But we have not observed
or measured it yet because at that place if
you would put a gravimeter, you would not see the signal because on a mountain you
have varying snow cover from summer and winter. The snow is changing, the
groundwater level is changing, and all these local changes and mass are disguising the actual effect. So if you would combine
now your gravimeter with the clock on top of such a mountain, then you really could resolve what the melting of the permafrost
is doing to the Alps because the clock would not be sensitive to a very local mass
change like snow cover. But it would really
measure an averaged mass and gravitational potential and really show you the
physical height of the mountain. That's why geodesy missions
using atomic clocks are expected to take place
in the next few years. So there's actually a mission in Germany, which geodesists want to make, for the first time, a clock measurement. They want to bring a clock to Heligoland - this is the offshore island
in the Northern Sea - to compare and get the data because at the moment this
island could not be leveled. It's too far out in the ocean and satellites would not resolve it. It's too small. And that's why there's
right now a geodetic mission to bring a portable atomic
clock to this island and compare it to a continental clock. Heligoland's location and size make getting precise
height readouts difficult. Using GPS with gravimeter data
requires multiple readouts every 2 kilometers, so offshore areas or areas near a shore technically require you to
place a gravimeter in the ocean, which isn't possible. That's why the island's elevation hasn't been accurately
measured to this day. Clocks can bridge the gaps to islands, deserts or mountainous areas, which are hard to access with
classical leveling methods. In turn, they can also act
as height reference points for understanding ocean
dynamics and global sea levels. But it's not all about geodesy. We may even learn things
about gravitational effects that we don't entirely understand. It's hard to predict what the
measurements will tell you, but certainly, we're up to many surprises. And this is actually the
boundary that we physicists are trying to push now, to measure more and more
precise time, frequency, space-time, as we know it's connected
in general relativity. Atomic clocks could be the key to answering some of the biggest
mysteries in physics today that shape the universe as
we know it, dark matter. We know that there's dark matter that's making up most of
the matter in the universe, and there are also models of dark matter that can be searched for and constrained using atomic clocks. So there are a lot of
scientific applications. As clocks get more precise, we could find that constants used in everyday
physics equations are wrong. And that could completely change how we understand the universe. Atomic clocks that we're building are the most sensitive way to look for this present-day variation in the fundamental constants. It really has the potential
to revolutionize physics. So do you think atomic clocks could lead to an Einstein-esque discovery about time? I think it's certainly possible. If we did detect that there was a drift in the fundamental constants over time, that would be a revolution similar to the discovery of relativity or the discovery of quantum mechanics. Basically, we're doing a lot of tests on the predictions of Albert Einstein because can we really assume that general relativity is the sole theory explaining everything or
is there maybe a deviation from the predictions that
general relativity makes? The fact is, I don't think we do understand time very well. It's always possible
that there is going to be some bright young person out there who's really terribly disturbed
by the conundrum of time and thinking it over and might come up with
something brilliant. But it's going to require a
different way of thinking. It's going to require
that flash a-ha moment that happens once per century
or even once per millennia.
Great video, thanks for sharing.